Method and apparatus for determining the progress of a uranium oxyfluoride conversion reaction in a furnace and for controlling the reaction

ABSTRACT

A substance in a condensed state, for example a powdered solid, is in continuous movement in the longitudinal direction ( 6 ) of a furnace ( 4, 5 ). A reactive gas mixture is brought into contact with the substance in the condensed state. A plurality of samples of the gaseous mixture are removed at a plurality of reference points ( 14 ) spaced apart from one another along the longitudinal direction ( 6 ) of the furnace ( 4, 5 ); each of the gas samples is analyzed outside the furnace to determine the composition of the gas mixture and for each point ( 14 ), the extent of a chemical reaction between the condensed substance and the reactive gas mixture is deduced from the composition of the gas mixture at each of the reference points ( 14 ). In particular, the apparatus comprises a sampling and injection rod ( 10 ) introduced into the furnace ( 4, 5 ) and disposed in its longitudinal direction ( 6 ). The invention is of particular application to modeling a rotary furnace ( 4, 5 ) for converting uranium oxyfluoride into uranium oxides and for controlling the conversion reaction in the furnace ( 4, 5 ).

FIELD OF THE INVENTION

The invention relates to a method and apparatus for determining theprogress of a chemical reaction in a furnace and for controlling thereaction. In particular, the invention is applicable to the productionof uranium dioxide powder used to manufacture nuclear fuel pellets.

BACKGROUND INFORMATION

In nuclear reactors, for example in pressurized water nuclear reactors,a fuel is used that can be constituted mainly of uranium oxide or of amixture of uranium oxide and plutonium.

Nuclear fuel, which is enriched in fissile elements, for example inuranium-235 in the case of a fuel constituted by uranium oxide, isgenerally obtained by a process in which the final enrichment product isconstituted by gaseous uranium hexafluoride UF₆.

The UF₆ is then converted into uranium oxide by oxidation using steam,for example.

Current processes producing the best results for converting uraniumhexafluoride into uranium oxide are dry direct conversion processeswhich are conducted in apparatus comprising, in succession, a reactorprovided with means for introducing UF₆ and steam into a chamber of thereactor in which uranium oxyfluoride UO₂F₂ is formed from the UF₆, and arotary tube furnace in which the solid powdered uranium oxyfluorideUO₂F₂ is transformed into uranium oxide, the tube furnace being providedwith heater means and means for introducing steam and hydrogen via theoutlet portion of the rotary furnace as a counter-current to thepowdered solid moving in the longitudinal direction of the furnace.

Uranium oxide powder principally constituted by the dioxide UO₂ isrecovered from the outlet of the rotary furnace, that powder then beingconditioned in a conditioning unit before being used to produce sinterednuclear fuel pellets.

The process for transforming uranium oxyfluoride UO₂F₂ into uraniumoxide is carried out by bringing a powdered solid into contact with areactive gas mixture containing steam and hydrogen in particular.

Hydrofluoric acid HF is formed in the rotary furnace by oxidizing thesulfur oxyfluoride with steam.

The solid material circulating in the rotary furnace coming into contactwith the reactive gas mixture is the seat of various chemical reactionsthat result in the formation of uranium oxide, and principally of thedioxide UO₂; in particular, the chemical reactions indicated belowoccur:UO₂F₂+H₂O→UO₃+2HF3UO₃→U₃O₈+½O₂U₃O₈+2H₂→3UO₂+2H₂O

Thus, the composition of the continuously moving solid material in thefurnace and the composition of the reactive gas mixture vary essentiallyin the longitudinal direction of the furnace along which the powderedsolid material moves, with the gas mixture moving as a counter-current.

In order to control the chemical reactions in the furnace to the bestpossible extent, the heater elements of the furnace disposed at theperiphery of the jacket of the rotary furnace are adjusted to obtain aregular temperature distribution in the longitudinal direction of thefurnace.

However, that method of adjusting the temperature in the longitudinalaxial direction of the furnace cannot effectively control thecomposition of the uranium oxides at the furnace outlet.

Adjusting the flow rates of the hydrogen and steam introduced via theoutlet end portion of the rotary furnace cannot improve control of theconversion reactions in the furnace because the reactive gases arediluted in the furnace and because of the random nature of thedistribution of the reactive gases obtained inside the furnace chamber.

Further, uranium dioxide UO₂ can be produced from the oxide U₃O₈ viaintermediate reactions during which different uranium oxides areobtained in accordance with the transformation sequenceU₃O₈→U₃O₇→U₄O₉→UO₂.

In general, no method is known for determining how the reactions betweenthe reactive gas mixture and the oxyfluoride or uranium oxides movingalong the rotary furnace are progressing in the longitudinal directionof movement of the substances inside the rotary furnace. Access to agraph of the progress of the reactions inside the rotary furnace wouldmean that the reactions could be manipulated to optimize the conversionprocess to obtain oxides with the desired composition at the furnaceoutlet.

In particular, in order to obtain green pellets with very highmechanical strength as measured by crush, microhardness or wear tests inwhich the uranium oxide powder is compressed, it has been observed thatit is necessary to use oxide powders of a composition such that theatomic ratio of the oxygen over the uranium (O/U) is substantiallyhigher than the ratios normally obtained with oxides from the outlet ofa uranium oxyfluoride converting furnace, which oxides are constitutedprincipally by uranium dioxide UO₂.

To increase the O/U ratio, mixing certain proportions of particles ofuranium dioxide UO₂ obtained by dry conversion with particles of anoxide such as U₃O₈ has been proposed, for example. That method, whichcan increase the O/U ratio of the oxides used to produce fuel pellets,necessitates oxidizing the uranium oxide UO₂ under perfectly controlledconditions in order to obtain the oxide U₃O₈, and then forming ahomogeneous mixture of UO₂ and U₃O₈ particles. Thus, that method ofproducing uranium oxide powders is complex.

Currently, no method is known that can control a reaction from anaccurate determination of the progress of a chemical conversion reactionin a furnace to obtain uranium oxides at the furnace outlet with thedesired composition, and in particular uranium oxides with a high O/Uratio, i.e., oxides with a mean formula of the type UO_(2+x), where x isrelatively high (x in the range 0.03 to 0.7, preferably in the range0.05 to 0.25).

More generally, when a chemical reaction is carried out between asubstance in a condensed state, for example a solid powdered substancemoving continuously in the longitudinal direction of a furnace, and areactive gas mixture, no method is known for accurately determining theextent of the reaction at different points along the longitudinaldirection of the furnace, and no method is known for controlling thereaction in the furnace from any such accurate determination.

An accurate determination of the extent of the reaction in the furnacemust be carried out without opening the furnace and without risking theintroduction of air into the furnace interior, since that would bothcompletely falsify the measurements and analyses carried out, and wouldalso modify the product being produced in the furnace. Thus, it is notpossible to monitor the extent of the reactions by removing samples ofthe moving powdered substance at different points along the axis andinside the furnace chamber.

SUMMARY

The aim of the invention is to provide a method of determining theprogress of at least one chemical reaction along the longitudinaldirection of a furnace, the reaction taking place inside a chamber ofthe furnace between a reactive gas mixture and a substance in acondensed state, for example a powdered solid, in continuous movement inthe longitudinal direction of the furnace, said method allowing theextent of the reaction and the progress of the chemical reaction in thelongitudinal direction of the furnace to be determined in a precise andaccurate manner, without needing to open the furnace and withoutmodifying the progress of the chemical reaction while implementing themethod.

To this end, a plurality of samples of the gas mixture are removed at aplurality of reference points spaced apart from one another along thelongitudinal direction of the furnace, each of the gas samples isexamined outside the furnace to determine the composition of the gasmixture, and the extent of the at least one chemical reaction is deducedfrom the composition of the gas mixture at each point.

The invention also provides a method of controlling at least onechemical reaction in a furnace to obtain a final product with apredetermined composition by using a prior determination of the progressof the chemical reaction in the longitudinal direction of the furnace tomodel the operation of the furnace. The chemical reaction is thencontrolled by injecting gases with a carefully selected composition andwith a predetermined flow rate at least one point into the interior ofthe furnace chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention, reference is now made to theaccompanying figures to illustrate carrying out the determination methodand the control method of the invention, in the case of a rotary furnacefor converting uranium oxyfluoride into uranium oxide.

FIG. 1 is a longitudinal section through a unit for converting uraniumhexafluoride into uranium oxide, comprising a rotary furnace providedwith a sampling and injection apparatus that enables the methods of theinvention to be carried out.

FIG. 2 is a perspective view of the sampling and injection apparatus ofthe invention.

FIG. 3 is a view of a central support portion of a sampling rod of theapparatus shown in FIG. 2.

FIG. 4 is a front view of a flange of the sampling and injectionapparatus shown in FIG. 2.

FIG. 5 is a cross-section on 5-5 of FIG. 4.

FIG. 6 is an axial section of a sampling and injection assembly of theapparatus shown in FIG. 2.

FIGS. 7A, 7B and 7C are diagrammatic views in three different functionalpositions of an analysis and purge circuit disposed outside the rotaryfurnace and connected to the sampling and injection apparatus of theinvention.

DETAILED DESCRIPTION

FIG. 1 illustrates a unit for converting uranium hexafluoride intouranium oxide, the unit being generally designated by the referencenumeral 1 and comprising a reactor 2 for converting uranium hexafluorideUF₆ into uranium oxyfluoride UO₂F₂ by injecting gaseous hexafluorideUF₆, steam, and nitrogen into the reactor 2.

Uranium hexafluoride UF₆ is transformed into the oxyfluoride UO₂F₂ byoxidation in steam in accordance with the reaction:UF₆+2H₂O→UO₂F₂+4HF

The uranium oxyfluoride UO₂F₂ produced by the reaction is in the form ofa powder that falls into the bottom of the reactor where this powder istaken up by a screw conveyor 3 for introduction into the inlet portionof a rotary furnace 4 inside which the uranium oxyfluoride powder isconverted into uranium oxide.

The furnace 4 comprises a rotatably mounted tubular jacket 5 driven inrotation about a longitudinal axis 6, which is inclined at an angle α tothe horizontal plane so that the inlet end 5 a of the rotary jacket ofthe furnace is disposed at a higher level than the outlet end 5 b.

Around the rotary furnace jacket inside an insulated chamber 7, heaterelements 8 are disposed that heat the internal volume of the rotaryjacket 5 in which the uranium oxyfluoride is transformed into uraniumoxide at high temperature in accordance with the following reactions:UO₂F₂+H₂O→UO₃+2HF  (1)3UO₃→U₃O₈+½O₂  (2)U₃O₈+2H₂→3UO₂+2H₂O  (3)½O₂+H₂→H₂O  (4)U₃O₈→U₃O₇→U₄O₉→UO₂  (5)

The steam and hydrogen required to carry out the conversion inside therotary furnace are introduced into the internal volume of the rotaryjacket 5, generally by an injection arrangement introduced via theoutlet end 5 b of the rotary jacket 5 of the furnace.

The uranium oxyfluoride powder UO₂F₂ introduced via the screw conveyor 3into the inlet end 5 a of the rotary jacket 5 is transported towards theoutlet end 5 b during rotation of the rotary furnace by dint of theslope α of the longitudinal axis of said jacket 5. Further, rotation ofthe furnace produces agitation and lifts the powder which then comesinto intimate contact with the steam and hydrogen injected into thefurnace jacket and which generally move as a counter-current to themovement of the powder inside the rotary jacket.

The end 5 b of the rotary jacket 5 opens into a chamber of a unit 11 forrecovering and conditioning the uranium oxide powder formed in thefurnace by conversion of the uranium oxyfluoride powder.

The different gases formed inside the furnace chamber by the reactionstransforming the oxyfluoride into uranium oxides and in particularhydrofluoric acid HF are recovered with excess hydrogen and steam whichhave not been used up in the chemical reactions occurring in thefurnace.

As explained above, one of the inherent problems with carrying out a drytype conversion process in a unit such as unit 1 arises from the factthat in general, no information is available regarding the progress ofthe conversion inside the furnace jacket and on the extent of thedifferent reactions occurring in the axial direction 6 of the rotaryfurnace.

As is explained below, accurate knowledge of the progress of thechemical reactions inside the rotary furnace enables the reactions to becontrolled by injecting reactive gases and in particular hydrogen insidethe rotary furnace at predetermined locations to obtain a uranium oxidemixture with general composition UO_(2+x), where x is fixed at apre-determined value, i.e., with an O/U ratio that is much higher than 2and fixed at a well determined value.

Up to now, different means have been used to influence the chemicalreactions inside the furnace, in particular by controlling the heaterelements to obtain an optimal temperature distribution inside thefurnace jacket along a longitudinal direction 6, and by the use ofbaffles 13 fixed inside the furnace to encourage contact between thereactive gas mixture moving in the furnace and the powder duringconversion.

Chemical reactions inside the rotary furnace may also be controlled byadjusting the flow rates of hydrogen and steam introduced via the outletend 5 b of the rotary jacket, generally via a fixed chamber of theuranium oxide powder recovery and conditioning unit. However,satisfactory control of the uranium oxides produced has never beenachieved by adjusting the flow rates of the steam and hydrogenintroduced via the outlet end of the furnace.

In accordance with the invention, the progress of the chemical reactionsoccurring inside the furnace is accurately determined using a rod 10 forsampling gas at different points distributed along the longitudinaldirection 6 of the furnace, connected to a gas analyzer 12 outside thefurnace.

The extent of the reactions listed above occurring inside the rotaryfurnace may be determined at each of the points in the furnace bymeasuring the proportion or partial pressure of the gases produced bythe various reactions or involved in said reactions, in particular HF,H₂, or H₂O. In particular, the presence or absence of a gas at areference point along the longitudinal direction 6 of the furnace showswhether a chemical conversion reaction is in progress or has beencompleted.

Before start-up of a conversion unit such as unit 1, a first modelingoperation is carried out that consists in causing the unit to functionunder its normal production conditions and in removing the gases presentin the furnace from a plurality of sampling points distributed along thelongitudinal direction 6 of the rotary furnace, then analyzing thesegases outside the furnace.

The results of the gas analyses at different points in the furnace areused to construct a model representing the progress of the differentchemical reactions occurring along the length of the furnace.

After this first stage of modeling the operation of the rotary furnace,the results of the modeling are used to determine the injectionsrequired at different points in the furnace, in particular hydrogeninjection to obtain a powder UO_(2+x) with an optimum O/U ratio from thefurnace outlet. In general, the O/U ratio is intended to be in the range2 to 2.7, preferably in the range 2.10 to 2.25.

It is possible to envisage producing oxides with an O/U ratio that is ashigh as 2.66, which has only been possible until now by mixing UO₂ andUO₃O₈ oxides.

To remove and analyze the gases inside the furnace (and optionally toinject gas into the furnace production phase), a long rod 10 is usedwith a length that is at least equal to the axial length of the jacket 5of the rotary furnace and which is introduced into the internal volumeof the rotary jacket 5 via its outlet end 5 b and is disposed along itslongitudinal axial direction 6. At one end of the furnace, the rod 10 isfixed to the rotary jacket of the furnace via an outer end portion to afixed portion such as the wall of the recovery chamber of the oxidepowder recovery and conditioning unit 11.

The sampling rod is connected to one or more gas analyzers 12 at its endlocated outside the furnace and the chamber of unit 11.

The rod 10 disposed in the axial longitudinal direction 6 of the rotaryfurnace 5 is completely free with respect to the rotary portions of thejacket 5 and in particular with respect to the baffles 13 occupying aportion of the cross section of the rotary jacket 5 of the furnace.

In the embodiment illustrated, the sampling rod 10 has ten samplingpoints 14 distributed along its length, to remove samples at tenreference points of the internal volume of the rotary jacket 5, with asubstantially constant spacing between two successive reference points.

Reference is now made to FIG. 2 to describe the sampling rod 10 assemblyof the invention.

The sampling rod 10 comprises a long central tube 15 on which there arefixed, in a coaxial manner with predetermined spacing in the axialdirection of the central support tube 5, support and guiding elements 16and 16′ which are primarily cylindrical in shape, each comprising, onits outer surface, a set of recesses each intended to receive onesampling tube 18 of a sampling tube assembly disposed parallel to thelongitudinal axis of the central tube 15.

In the embodiment illustrated, each of the guiding and support elements16 and 16′ comprises ten recesses around its periphery in the shape ofrectilinear channels of cross section having a semi-circular portion,machined into the support element 16 or 16′.

Fixed on the central tube 15, between two support elements 16 andequidistant from the two support elements 16, there is an intermediatesupport element 16′ which is shorter in the axial direction than are thesupport elements 16.

The support elements 16′ comprise a plurality of recesses of number andcross-section that are identical to those of the recesses in the longersupport elements 16.

One axial end of the central tube 15 of the rod 10 is fixed to a flange17 for connecting the rod 10 to the fixed wall of the unit 11, at theoutlet from the rotary furnace.

The flange 17 has tapped openings for fixing the flange 17 and the rod10 to the wall of the unit 11, so that the rod 10 is in a position thatis coaxial with the jacket 5 of the rotary furnace, i.e., with the axisof its central tube 15 along the longitudinal axis 6 of the jacket 5 ofthe rotary furnace.

The end of the central tube 15 opposite from the end connected to theflange 17 is fixed to a pad 19 for closing the end of the central tube15 which is inside the furnace, in a position close to the inlet end 5 aof the rotary jacket 5.

The face of flange 17 that is opposite in the axial direction to theface thereof connected via an extension 21 to the central tube 15 of therod is integral with a sampling assembly 22.

Each small diameter sampling tube 18 passes through a portion of theflange 17 in the axial direction, then is bent at 90° in a radialdirection for connection, at the outer periphery of the flange 17, to anelement 23 for connecting the tube 18 to an extension piece 18′ thatplaces the tube 18 in communication with a valve housing 24′ of a valve24 of the sampling apparatus 22.

Each valve housing 24′ of a valve 24 of the sampling apparatus 22 isconnected, via a curved connecting tube 18″, to a sampling chambermachined in the housing of the sampling apparatus 22.

Each tube 18 may be placed in communication with the chamber of thesampling apparatus 22 via tubes 18′ and 18″ and a valve 24, 24′.

Each tube 18 extends in the axial direction of the rod 10 between theflange 17 and a sampling point 14 corresponding to a reference pointinside the rotary furnace 4,5 at or near a short support and guidingelement 16′.

FIG. 3A illustrates the central tube 15 of the sampling rod 10 on whichlonger tube support and guiding elements 16 are fixed at regularintervals with shorter support and guiding elements 16′ interspersedbetween the longer support and guiding elements 16.

A shorter guide element 16′ intended to receive the last tube 18 thatcarries out the sampling at the last sampling point 14 located near theinlet end 5 b of the rotary jacket of the furnace is fixed after thelast longer guide element 16 on the end portion of the central tube 15of the sampling rod 10, which is introduced into the rotary jacket ofthe furnace until it is close to the inlet end of the rotary jacket.

The end portion of the central tube 15 opposite to the end located nearthe inlet portion of the rotary jacket of the furnace has no support andguiding elements 16 or 16′ along a section having an end which isintended to receive the flange 17 for fixing the sampling rod on thefixed structure of the unit.

The end portion of the rod comprising the end section of the centraltube 15 that is free of guiding elements 16 and 16′ is engaged throughthe chamber of the powder recovery and conditioning unit 11 and in theoutlet portion of the rotary jacket 5 when the sampling rod 10 is fixedin its working position. In this portion of the sampling rod, thesampling tubes 18 located at the periphery of the central tube 15 arenot fixed to the outer wall of the central tube 15.

FIG. 3B is a cross-section through a support and guiding element 16comprising ten recesses 26 each for receiving one sampling tube 18 andformed in the shape of channels with semi-circular bases extending inthe axial direction over the peripheral surface of the support andguiding element 16 with a primarily cylindrical shape.

The section of the short support and guiding elements 16′ is identicalto the section of the longer support and guiding element 16 shown inFIG. 3B.

FIGS. 4 and 5 illustrate the flange 17 on which one end of the centraltube 15 of the rod 10 is fixed and which forms the connection betweenthe second end portions of the sampling tubes 18 and the extensions 18′of each of said sampling tubes.

One face of flange 17 in the axial direction comprises a projectingportion 17 a on which the end of the central tube 15 is engaged andfixed by welding.

The sampling tubes 18 disposed about the central tube are each engagedin an opening passing through the axial direction of the flange 17opening into a cylindrical cavity on the second face of the flange 17.

The openings for the tubes 18 through the flange 17 are disposed in theform of a circular array in the central portion of the flange 17 aroundthe projecting portion 17 a.

Tapped openings 25 for fixing the flange 17 onto a fixed portion of theconversion unit, for example on the wall of the chamber of the powderrecovery and conditioning apparatus, are formed in the outer peripheralportion of the flange 17.

The end portion of the sampling tubes 18 is bent at 90° to place it in aradial direction with respect to the flange 17 and is engaged in anopening in the flange 17 opening into a chamber 27 for connecting theend of the sampling tube 18 with an S-shaped extension 18′.

A connecting chamber 27 is provided for each of the ten sampling tubes18, around the outer periphery of the flange 17, each of the chambers 27being closed by a plug.

The radially bent portion of the tube 18 and S-shaped extension 18′connect the sampling tubes surrounding the central tube 15 in a circularline of small diameter to the housings 24′ of valves 24 of the samplingapparatus 22 disposed in a circular zone of diameter greater than thediameter of the flange 17.

FIG. 6 illustrates that the sampling apparatus 22 comprises a housing ofright prismatic shape 28 on which is fixed, in a coaxial disposition,the extension 21 that is integral with a plug 29 intended to be engagedin and fixed into the cavity machined on the second face of the flange17. This provides the connection between the sampling apparatus 22 andthe flange 17.

The sampling apparatus 22 comprises ten valves 24 having housings 24′which are fixed one after another in the circumferential directionaround the housing 28, which preferably has a prismatic shape and adecagonal cross-section.

On the face opposite to the face connected to the connecting extension21 of the flange 17, the housing 28 of the sampling apparatus 22comprises a cavity 30 that is partially closed on the outer face of thehousing 28 by an annular cover 31 with an internal bore that sealinglyreceives a tube 32 communicating with a connector 33 of the samplingapparatus. The cavity 30 has the smallest possible volume to reduce theinertia of the sampling system and to limit the risk of dilution of themixture, and it constitutes a sampling chamber 34 with the internalspace of the tube 32.

Each of the valve housings 24′ of the valves 24 is connected to thesampling chamber 34 via a respective connection tube 18″. One end ofeach bent connection tube 18″ is connected to the housing 24′ of a valve24 and its second end is connected to an opening in the cover 31 closingthe cavity 30. Each of the valve housings 24′ of a valve 24 comprises afirst chamber to which an extension tube 18′ is connected and a secondchamber to which a connection tube 18″ is connected. When the valve 24is closed, the two chambers are separated by the valve obturator, whichmay be a bellows-sealed valve. On opening valve 24, the two chambers ofthe valve housing and the sampling tube 18″ are placed in communicationwith the sampling chamber 34 via the connection tubes 18′ and 18″.

As may be seen in FIGS. 7A, 7B and 7C, the sampling chamber 34 isconnected via the connector 33 to a gas purge, analysis, and injectioncircuit 35.

The circuit 35 comprises at least one line 36 connecting the connector33 of the sampling chamber 34 and at least one gas analyzer 37, via ashut-off valve 38 and a three-port valve 39.

The three-port valve 39 has one port connected to the sampling chamber34 via the connector 33 and a second port connected to the analyzer 37via the line 36, and in a first position shown in FIG. 7A, it may placethe sampling chamber in communication with the gas analyzer 37 when theshut-off valve 38 is open.

The principal sampling line 36 is connected via the third port of thethree-port valve 39 to a side line 41 connected to a reservoir 40 thatmay contain an inert purge gas such as nitrogen, argon, or helium, or areactive gas such as hydrogen.

As illustrated in FIG. 7B, when three-port valve 39 is placed in theposition illustrated and the valve 38 is closed, analyzer 37 may bepurged with purge gas from reservoir 40.

When three-port valve 39 is placed in its position shown in FIG. 7C andstop valve 38 is open, a purge gas may be sent into at least one ofsampling tubes 18 via sampling chamber 34.

When reservoir 40 is a reservoir for a reactive gas such as hydrogen,the reactive gas may be sent into at least one sampling tube 18 via thesampling chamber 34, the three-port valve 39 being in its position shownin FIG. 7C and the shut-off valve 38 being open.

By actuating both the valves 24 of the sampling and distributionapparatus 22, which valves are preferably solenoid valves, and also thecircuit valves 35, it is possible to purge the gas analyzer, purge anysampling tube 18 or a plurality of sampling tubes, or inject thereactive gases such as hydrogen into the rotary furnace of theconversion unit, at any reference point 14 or at a plurality ofreference points in the furnace chamber.

As explained below, the apparatus described may make it possible toremove samples of gas at a plurality of points distributed along thelongitudinal direction of the furnace and to analyze gas samples undervery good conditions, enabling a graph of the progress of the chemicalreactions in the furnace to be produced and thus providing a model ofthe furnace for the production of uranium oxide.

From a model of the oxide production furnace, it is possible todetermine the injections of reactive gases, in particular hydrogen,required to obtain uranium oxide with a mean composition UO_(2+x) withan O/U ratio of a desired value at the furnace outlet.

The reactive gases may be injected at one or more reference points inthe furnace chamber each corresponding to a sampling point 14 by openingone or more valves 24 of the sampling and injection apparatus 22.

It is also possible to inject reactive gas at each reference point inthe rotary furnace by opening all of the valves 24.

To carry out the gas sampling and analysis phase in the rotary furnace,the analyzer 37 is first purged with an inert gas then a first samplingline is purged. The reactive gas at the corresponding reference point 14in the furnace is then sampled using the sampling tube, which haspreviously been purged. The gas samples are sent to the gas analyzer 37which provides the composition of the sampled reactive gas mixture,i.e., the concentration or partial pressure of gases such ashydrofluoric acid HF and/or steam in the mixture and/or hydrogen and/ornitrogen.

To carry out an analysis of a sample that is perfectly representative ofthe atmosphere in the furnace in the sampling zone under consideration,it is necessary to ensure that the gas mixture is not modified by aninternal reaction by deposition of substances or by condensation of thesampled gas.

To prevent further chemical reaction in a gas sample between thesampling point and the analyzer, the gas mixture is filtered at theinlet to the sampling tube by passing it through a metal filter made ofa material that is resistant to the atmosphere of the furnace, forexample a nickel alloy, to stop any solid material particles that may bein suspension in the gas sample. This prevents the chemical reactionfrom continuing by removing one component.

To avoid any condensation or deposition in the removed gas sample, theportions of the sampling tubes that are located outside the furnace andconnected to the analyzer are heated. The valves 24 of the samplingapparatus 22 are also heated.

It is possible to use a single analyzer and thus a single line formoving the gas samples between the sampling apparatus 22 and theanalyzer 37 or, in contrast, a plurality of analyzers connected to thesampling apparatus 22, each of the analyzers, for example, assaying oneof the gases in the reactive gas mixture removed from the furnace.

Thus, in a first furnace modeling phase, the method and apparatus of theinvention may remove gas samples from the furnace and analyze said gassamples, so that the result of the analyses is completely representativeof the composition of the reactive gas mixture removed at the samplingpoints inside the furnace.

It is therefore possible to obtain a very accurate model of the rotaryconversion furnace by determining the extent of the chemical conversionreactions at each of the furnace sampling points, more particularly thedegree of completion of said chemical reactions. The model is in theform of graphs of the progress of the chemical reactions for formingoxides along the longitudinal direction of the furnace.

As an example, it is possible to monitor the concentration of HF or H₂in the furnace in its longitudinal direction and to deduce the extent ofthe uranium oxyfluoride transformation reactions.

From the modeling of the furnace, it is possible to accurately determinethe reference points in the furnace chamber where reactive gas such ashydrogen must be injected along with the rate at which it should beinjected, and thus to steer the chemical reactions so as to obtain auranium oxide with the desired mean composition at the furnace outlet.

This mean composition is then obtained without mixing oxide powders withdifferent compositions, the powder leaving the furnace having thedesired composition.

Further, injecting reactive gases directly into a zone of the furnacewhere said reactive gases are required may prevent a deficiency in thereactive gases, a disadvantage that arises when injecting reactive gasesvia the end of the rotary furnace. The reactive gases are not diluted bythe furnace atmosphere and are introduced into the precise locationwhere they have to be used, which limits the quantities of reactivegases used. Thus, the cost of producing uranium oxide is reduced.

The gases are removed from the furnace without using a suction orpumping arrangement because the pressure of the furnace exceeds that ofthe outer atmosphere. When a valve in the sampling apparatus is opened,the reactive gas then flows at high speed into the sampling tube 18which is of section that is sufficiently small for the flow of theremoved gas to remain very limited. In this way, samples are obtained ofa composition that is entirely representative of the composition of theatmosphere in the furnace at the sampling points.

When converting uranium oxyfluoride into uranium oxide, assayinghydrogen in the samples may in particular allow determination of whereand by how much the oxide UO₂ is being formed by reduction. It is thenpossible to influence the reactions to displace the point for totaltransformation into UO₂ to modify the composition of the oxides and theO/U ratio.

Assaying the hydrofluoric acid HF in the samples allows the uraniumoxyfluoride to oxide transformation to be monitored.

The invention is not limited to the embodiment described.

Thus, it is possible to sample and inject gas in the furnace usingarrangements other than those described.

It is possible to remove samples of the furnace atmosphere and to injectgas at any number of points. As an example, it is possible to implementthe invention in a uranium oxyfluoride conversion furnace by injectingat five points of the ten sampling points.

The nature and flow rates of the injected gases depend on the nature ofthe chemical reactions occurring in the furnace and on the flow rates ofthe moving substances.

The invention is not limited to furnaces for converting uraniumoxyfluoride into uranium oxide but may encompass applications in aplurality of furnaces in which a substance moves in its dense form, forexample a powder, a paste or even a liquid form, which comes intocontact with a reactive gas mixture and which therefore undergoestransformations in said furnace by chemical reactions.

The invention is also applicable outside the field of powder productionfor the manufacture of nuclear fuel.

1. A method of determining progress of at least one chemical reactionalong a longitudinal direction of a furnace, the at least one chemicalreaction taking place inside a chamber of a furnace between a reactivegas mixture and a substance in a condensed state in continuous movementin the longitudinal direction of the furnace comprising: removing aplurality of samples of the gas mixture at a plurality of referencepoints spaced apart from one another along the longitudinal direction ofthe furnace; analyzing each of the gas samples outside the furnace todetermine a composition of the gas mixture; and deducing an extent ofthe at least one chemical reaction from the composition of the gasmixture at each point wherein the at least one chemical reaction is fromcontacting uranium oxyfluoride in a powder form moving in the furnacewhich is rotary with the reactive gas mixture to convert the uraniumoxyfluoride into uranium oxides and the deducing the extent of the atleast one chemical reaction is from the composition of the gas mixtureat each of the reference points to obtain a model of the furnace in aform of graphs of a progress of uranium oxide formation reactions in thelongitudinal direction of the furnace.
 2. The method according to claim1, further comprising: determining a concentration of hydrofluoric acidin the gas samples removed from the rotary furnace.
 3. The methodaccording to claim 1, further comprising: determining a concentration ofat least one of hydrogen, nitrogen and steam gases in the gas samplesremoved from the rotary furnace.
 4. A method of controlling a conversionof uranium oxyfluoride in a powder form moving in a longitudinaldirection of the rotary furnace wherein the uranium oxyfluoride is incontact with a reactive gas mixture to convert the uranium oxyfluorideinto uranium oxides comprising: producing a model of a rotary furnace ina form of graphs of a progress of uranium oxide formation reactions inthe longitudinal direction of the furnace by deducing an extent of atleast one chemical reaction from a composition of the gas mixture atreference points; and injecting the reactive gas mixture with at leastone reactive gas into the rotary furnace at least one reference pointdefined from the model by a flow rate of the furnace model and physicalproperties of the gas to obtain uranium oxides with a mean compositionof and with a predetermined O/U ratio at an outlet of the rotaryfurnace.
 5. The method according to claim 4, wherein the predeterminedO/U ratio of the uranium oxides at the outlet from the rotary furnace isin a range of 2 to 2.7.
 6. The method according to claim 4, wherein thepredetermined O/U ratio of the uranium oxides at the outlet from therotary furnace is in a range of 2.10 to 2.25.
 7. An assembly comprisinga rotary furnace for converting uranium oxyfluoride in a powder formmoving in a longitudinal direction of the furnace into uranium oxides bycontacting the uranium oxyfluoride with a gas mixture, said assemblyalso comprising: an apparatus for carrying out a method for determiningprogress of the conversion of uranium oxyfluoride along the longitudinaldirection of the furnace, said apparatus comprising: a sampling andinjection rod comprising a central support tube; an arrangementconfigured to fix the central support tube inside the furnace in adisposition parallel to the longitudinal direction of the furnace; aplurality of sampling tubes positioned parallel to an axis of thecentral support tube distributed around a periphery of the centralsupport tube; and a sampling device disposed outside the furnaceconnected to at least one gas analyzer, each of the sampling tubescomprising a first end opening into the furnace at a sampling point anda second end connected to the sampling device having valves to connectone of, in succession and in a grouped manner, each of the samplingtubes to the at least one analyzer wherein the sampling rod is fixed toone end of the rotary furnace, said first ends of the sampling tubesbeing distributed along the longitudinal direction of the furnacewhereby an extent of the conversion of uranium oxyfluoride can bededuced from the composition of the gas mixture at the first ends of thesampling tubes.
 8. The assembly according to claim 7, furthercomprising: a plurality of support and guiding elements with a generallycylindrical shape are fixed in a coaxial disposition around the centralsupport tube at positions spaced along the axis of the central supporttube, each of the support and guiding elements comprising a plurality ofrecesses each intended for one sampling tube of the plurality ofsampling tubes.
 9. The assembly according to claim 7, wherein thesampling device disposed outside the furnace comprises: a housingconnected to the central tube of the sampling rod via an element forfixing the sampling rod inside the furnace, a sampling chamberconfigured in the housing of the sampling device; and a plurality ofvalves distributed around the housing of the sampling device each valvecomprising a chamber in two parts separated from each other by anobturator of the valve, the sampling tubes each connected at a secondend to a first chamber of a valve housing and a connector tubeconnecting a second chamber of the valve housing to the samplingchamber, the sampling chamber connected to the at least one analyzer viaat least one line configured with at least one valve.
 10. The assemblyaccording to claim 9, wherein the sampling chamber of the samplingdevice is connected to the at least one analyzer via a line comprisingin succession, between the chamber and the at least one analyzer, ashut-off valve and a three-port valve having one port connected to aportion of the line communicating with the sampling chamber, a secondport connected to a portion of the line connected to the at least oneanalyzer and a third port connected to a reservoir containing one of aninert purge gas and a reactive gas.
 11. The assembly according to claim7, further comprising: a filter placed at an end of each of the samplingtubes, the filter configured to stop condensed substances and configuredto stop the at least one chemical reaction in the sampling tube betweenthe sampling point and the at least one analyzer.
 12. The assemblyaccording to claim 7, further comprising: an arrangement to heat thesampling tubes and valves of the sampling device to prevent condensationof a gaseous substance in the sampling tubes between the sampling pointsand the at least one analyzer.
 13. The assembly according to claim 7,wherein the arrangement for fixing the central tube of the sampling rodis configured with a flange configured to allow passage for each of thetubes for connection to the sampling device.